Rett Syndrome (RTT) is a severe neurological disorder in humans that results from mutations in the gene encoding methyl-CpG binding protein 2, MeCP2. One of the mysteries surrounding RTT is that although MeCP2 is a ubiquitous protein, the manifestations of the disease are restricted primarily to the central nervous system. Possible explanations have included increased sensitivity of neurons to loss of MeCP2 and lack of redundancy of MeCP2 function in neurons. Here we present an alternative idea, that the nervous system manifestation of RTT is due, at least in part, to loss of MeCP2 from glia, which in turn results in a negative influence on neurons. This idea is based on our preliminary findings that 1) MeCP2 is expressed in glia as well as in neurons, 2) that astroglia from RTT mice exhibit chromatin modifications similar to those in brains of mice lacking MeCP2, suggesting a less repressive chromatin status, and 3) that, in co- culture, RTT astroglia confer an aberrant morphology on wild type neurons. We will test this new idea in three specific aims. First, we will identify the defects induced in wild type (WT) neurons by astroglia from RTT mice and determine, conversely, whether defects in neurons from RTT mice are rescued by WT astroglia. Second, our conditioned medium experiments suggest that astroglia from RTT mice secrete a soluble protein inhibitory to neuronal morphology. We will analyze by microarray and biochemical approaches, the nature of the aberrant factor(s) secreted by astroglia from RTT mice. Finally, we will characterize RTT symptoms in a newly generated mouse model in which MeCP2 can be deleted either specifically from astroglia or from both neurons and astroglia. These mice will provide in vivo confirmation of a role for MeCP2-null astroglia in conferring aberrant neuronal morphology in RTT and permit functional studies of neurons in the context of abnormal astroglia. The proposed studies offer the potential for a new attack, through pharmacological inhibition of potential glial secreted inhibitory factors, on this debilitating neurological disorder that strikes one in 10,000-15,000 girls. PUBLIC HEALTH REVELANCE This grant identifies, for the first time, a potential role for a factor secreted from glia as an underlying cause of neuronal damage in Rett Syndrome. If, as our data suggest, the glial effects are due to secretion of an inhibitory growth factor, it raises the possibility of therapeutic intervention for treatment of RTT patients.